In the fields of digital mobile communications and speech storage, a speech coder is in use which compresses speech information and encodes compressed speech information at low bit rates for effective utilization of radio waves and storage media. In this case, when an error occurs in the transmission path (or recording media), the decoding side detects the error and uses an error compensation method to suppress deterioration in the quality of decoded speech.
Examples of such a conventional art include an error compensation method are described in a CS-ACELP coding system of the ITU-T Recommendation G.729 (“Coding of speech at 8 kbit/s using conjugate-structure algebraic-code-excited linear-prediction (CS-ACELP)”).
FIG. 1 is a block diagram showing a configuration of a speech decoder including error compensation according to the CS-ACELP coding system. In FIG. 1, suppose speech is decoded in 10 ms-frame units (decoding units) and whether any error is detected or not in the transmission path is notified to the speech decoder in frame units.
First, the data received and coded in a frame in which no transmission path error has been detected is separated by data separation section 1 into parameters necessary for decoding. Then, using lag parameters decoded by lag parameter decoding section 2, adaptive excitation codebook 3 generates adaptive excitation and fixed excitation codebook 4 generates fixed excitation. Furthermore, using a gain decoded by gain parameter decoding section 5, multiplier 6 performs multiplications and adder 7 performs additions to generate an excitation. Furthermore, using LPC parameters decoded by LPC parameter decoding section 8, decoded speech is generated via LPC synthesis filter 9 and post filter 10.
On the other hand, with respect to the data received and coded in a frame in which some transmission path error has been detected, an adaptive excitation is generated using the lag parameter of the previous frame in which no error has been detected as a lag parameter, and a fixed excitation is generated by giving fixed excitation codebook 4 a random fixed excitation code and an excitation is generated using a value obtained by attenuating the adaptive excitation gain and fixed excitation gain of the previous frame as a gain parameter, and LPC synthesis and post filter processing are carried out using the LPC parameter of the previous frame as an LPC parameter to obtain decoded speech.
In the event of a transmission path error, the above-described speech decoder can perform error compensation processing in this way.
However, since the above-described conventional speech decoder carries out same compensation processing irrespective of speech characteristics (voiced or unvoiced, etc.) in a frame in which an error is detected and carries out error compensation primarily using only past parameters, there are limits to improvement of deterioration in the quality of decoded speech during error compensation.